WATER CONTENT OF PLANT AND SOIL 21 



11. THE RELATIVE AMOUNT OP WATER IN THE 

 PLANT AND IN THE SOIL. 



1. The Amount of Water in Healthy Plants and in the Soil 

 in Which They Chow. 



Most plants contain a relatively high per cent of water in 

 their tissues. Hydrophytes, as a rule, are the most watery 

 and xerophytes the least so ; yet in the latter case there are 

 exceptions among succulents. Mesophytes, as other plants, 

 vary in water content according to the per cent of woodiness 

 or of succulency. Seedlings contain a higher per cent than 

 plants nearer to maturity. The per cent of water usually 

 decreases gradually until maturity is reached, then rapidly 

 to the minimum of the death point. 



There are several physical conditions, which may affect 

 the water-retaining power of the plant. Among these are 

 varying humidity and soil moisture, excessive or deficient 

 light, poor food supply, injurious chemicals in the soil, and 

 injurious gases in the air or soil. Varying soil composition 

 does not seem to affect the water content of the plant unless 

 it causes a lack of food or water, or unless there are present 

 substances which act as poisons to protoplasm. 



The following table gives the per cent of water in seedling 

 plants and in the soil at the same time. The plants were 

 three weeks old and were under the most favorable condi- 

 tions in the plant-house during the month of May. The 

 amount of light received by these plants was considerably 

 less than that received by plants under natural conditions. 

 (For record of physical factors see Comparative tlot III.) 



